To determine the moisture content in a sample, the sample is dried and the weight of the sample is measured before and after the drying process. Due to the extensive amount of work involved, this method is very expensive as well as error-prone.
In some cases, the weight loss can also be measured during the drying process. In a given sample, the decrease in weight is a function of the temperature, the length of the drying time, and the conditions in the test compartment, and it conforms to a weight-versus-time curve which asymptotically approaches the dry weight of the sample. The curve for the given sample is determined by comparative experiments and can be expressed mathematically through an approximation formula. A measuring instrument for gravimetric moisture determination which is appropriately equipped with available electronic technology can compute the moisture content of a sample based on the measured parameters of the aforementioned curve and based on the length of the drying time and indicate the result on a display unit. With this method, the substance to be dried does no longer need to be totally desiccated; it is sufficient to determine the coordinates of two measurement points in the weight-versus-time diagram.
As has already been mentioned at the beginning, the weight change of a sample is substantially a function of the temperature, the length of the drying time, and the conditions in the test compartment. Especially the stringent requirements imposed on the test compartment are setting a limit to the accuracy of the commercially available instruments.
The term “test compartment” in the present context means a space which is enclosed by the housing of the measuring instrument and which can be opened in order to insert or remove a sample. Also arranged inside the test compartment are a sample receiver and a means to heat the sample. The sample receiver is connected to a gravimetric measuring instrument.
Normally, the sample is spread in a thin layer onto a flat sample receiver, for example a sample tray. For a uniform heating of the sample, the sample tray is preferably positioned so that its flat area is horizontal and parallel to the planar area occupied by the sample-heating means. As a means for heating the sample, a variety of radiation sources are used, such as heat radiators, microwave generators, halogen- and quartz lamps.
For a reliable drying process, it is necessary that the vapors coming out of the sample during the measuring process can be removed from the test compartment. It is therefore normal practice to arrange vent openings in the housing of the measuring instrument between the test compartment and the ambient environment of the measuring instrument. The vent openings which are arranged in the lower part of the test compartment serve as inlets for relatively dry and cool air. As the air is heated in the test compartment by the radiation source, it rises up inside the test compartment and is able in its heated state to absorb the moisture from the sample. The warm air that is charged with moisture leaves the test compartment through the vent openings that are arranged in the upper part of the test compartment.
A gravimetric moisture-determination instrument of the aforementioned type is disclosed in the U.S. Pat. No. 6,255,603 B1, issued to Spannagel on 3 Jul. 2001. The housing of this instrument has vent openings above the radiation source. These vent openings allow moisture from the sample to escape. Depending on the sample whose moisture content is to be determined, it is possible that as a result of the heating, other volatile substances are driven out of the sample, which may for example have a strong odor of their own or could be toxic or caustic. Such volatile substances can be produced in particular by a partial thermal decomposition of the sample. Furthermore, the moisture may not be limited to water that is distributed in the sample, but it is also possible that volatile substances whose boiling point is lower than the boiling point of water are driven out of the sample during the measuring process. This includes for example organic and inorganic solvents, gases that are occluded or dissolved in the sample, plasticizers that escape when testing plastic materials, and similar substances.
As mentioned above, warm air rises from the bottom to the top through the test compartment. Part of the rising air sweeps along the sample receiver and thereby causes a force acting against the direction of the load. Since this force depends to a large part on the velocity of the air flow in the test compartment, but as it also continuously changes due to the variable conditions in the test compartment, it is very difficult to compensate the error that this force causes in the weighing result. The drying rate also varies along with the air flow velocity, as the removal of the moisture escaping from the sample significantly affects the drying process.
Due to the errors in the time data which occur as a result, the accuracy that can be achieved in an analysis according to the mathematical model described above is limited. As an alternative to using the mathematical model, one can resort to the known method in which all of the moisture has to be driven out of the sample, at least to the extent that this is possible. However, this requires a very long drying time, whereby the risk is increased that a thermal decomposition or oxidation of the sample will occur as a result of the extended exposure to the heat from the radiation source.
For the reasons that have just been explained, it is hardly possible to determine an absolute value for the moisture content with a gravimetric moisture-determination instrument. For a more accurate determination of the moisture content of a substance, the Karl Fischer titration method is therefore still in use. This method is very labor-intensive, prone to user errors, and expensive.
It is therefore the object to provide in a gravimetric moisture-determination instrument of the kind mentioned in the introduction a test compartment with improved test conditions, in which the moisture content of a sample can be determined more precisely.